Time Travel Paradox

Yeah, in weak sci-fi But the usual way is "consistency". In General Relativity, any possible space-time is a full 4d entity: past, present, future. You cannot start with some fixed past and generate arbitrary futures. The space-time comes as the whole package. Does wonders for thoughts of pre-destination and free-will.A paradoxical scenario as described by Guido is not a valid space-time... hence it can't happen. Only consistent space-times are allowed.By the way, we actually have a name for Guido's experimental set-up: wormhole billiards, the subject of my very first research. One really intersting part of this is if we fix the initial and final conditions, say ball rolls towards the in-hole, and ball rolls away from out-hole, then classically there should only be one solution.But with the time-machine aspect, we get an infinite number of solutions which we must sum over to get some average: ball goes in in-hole, comes out of out-hole; ball goes in in-hole, comes out out-hole, goes back in in-hole and finally come out of out-hole again; ball isn't going in, but collides with it-self, knocking it in... it comes out, just in time to make the collison with itself and heads out; etc, etc, etc.This is suspiciously like Feynman sum-over-histories quantum mechanics...Edited by cavediver, : No reason given.

Nice thought experiment Check out my reply to CK above. We wait for the "time-lines to adjust" whenever I hear that in Star Trek (all series) I reach for my gun...Edited by cavediver, : No reason given.

Well, the obvious answer is that they don't and won't until we have a more concrete idea of quatum gravity.However, I personally do not believe* in quantum probabilities as such. QM is wholly deterministic and the probabilities only come from this bodge known as wave-function collapse/state-vector reduction. My vote is with decoherence, where the classical regime is reached through wave-functions evolving into more and more sharply peaked "classical" distributions through countless yet deterministic interactions with the environment. The graviton background may be a major player in this environment. The QM probabilities are just averages over these unobserved environmental interactions. Not at all! Wormholes as we now know them were discovered by Misner and Thorne by the direct request of Sagan whilst he was writing Contact! He needed a reasonable FTL scenario. This spawned a spate of papers in the mid to late eighties.Any mechanism that allows efective FTL travel has the capacity to form Closed Time-like Curves (CTCs), commonly known as backwards time-travel.*as in, 75% of the time I don't believe, and 25% I do Only a fool would ever be definite about any of this stuff...

There is a theoretical version of it known as Alcubierre’s Warp Drive, bt it's not something you would carry on a ship! It would probably require some roped off corridor of space between to fixed points to work properly. Not that OT. As I mentioned in the post above, any FTL mechanism is automatically a time machine and will give rise to Guido's original concerns.Don't try to follow Star Trek to closely on its physics. Voyager could have been home in a week, but it would have been difficult squeezing in 5 or more series

Not necessarily. Any time-machine cannot send you back earlier than its own first moment of existence. So, you cannot build a time-machine to go back in time (sorry!), but can build one to return to that time.The absence of time-travellers only suggests that we haven't managed to exploit naturally occuring or alien time machines. Or as you say, the time cops are good...The boring answer is that time travel, although theoretically possible with our current understanding, is practically impossible. Every time-machine envisioned is dogged by problems, sufficiently so that Hawking suggested the Chronology Protection Conjecture (CPC) that states the universe contrives to make (backwards) time-travel impossible.

This is precisely the way we turned single wormholes into time machines back in the eighties.If one objects to this, you can always use two "fixed" wormholes.

Hey, if you're clever enough to make one, you should have no trouble moving it Wormhole stability is a big question in itself, before you even start thinking of moving it. You have to prop it open with what we call exotic matter, effectively negative energy density. The Casimir effect is one way of generating this. However, since the time I working on wormholes, we have discovered the universal expansion is accelerating, driven by such a negative energy density.